This application claims priority to Korean Patent Application No. 2004-7825, filed on Feb. 6, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to data storage devices such as HDDs (hard disc drives), and more particularly, to compensating for jitter in servo signals by adjusting a start time of a servo gate pulse in real time.
2. Description of the Related Art
Conventional art related to the present invention is disclosed in U.S. Pat. No. 6,069,764 and Japanese Patent Publication No. 2000-036164. U.S. Pat. No. 6,069,764 provides a technology that compensates for repeatable run out (RRO) errors by determining RRO values in time domain and injecting the RRO values into a servo loop. Japanese Patent Publication No. 2000-036164 provides a technology that outputs and stores a standard run-out correction coefficient, and generates a precise position error signal using the coefficient.
A hard disk drive (HDD) is an example data storage device within a computer system that reproduces data from disks or writes data onto the disks using magnetic heads. Higher data capacity, higher data density, and smaller size of the HDD result in higher bits per inch (BPI) and higher tracks per inch (TPI). BPI indicates recording density in a rotation direction of a disk, and TPI indicates recording density in a radial direction of a disk.
Such higher recording density results in higher number of tracks. More time is spent for recording servo information onto disks with such higher number of tracks during manufacturing the HDD.
For avoiding such time consumption, an offline servo track write (offline STW) method is used for loading a disk containing servo information into a HDD. In this method, writing the servo information onto the disk within the HDD is omitted. Rather, the disk having the servo information written in advance thereon is loaded into the HDD.
Generally, the servo information is written around the tracks of a disk 12. Referring to FIG. 1A, solid circular lines 100 indicate the tracks around the disk 12 about a center point O. However, such a center point O may not be aligned with a center O′ of the spindle motor of the HDD. The dashed line 102 indicates a path of a head of the HDD during rotation about the center O′ of the spindle motor.
Further referring to FIG. 1A, the misalignment of the track 100 and the rotational path 102 is referred to as eccentricity of the disk. FIG. 1B illustrates the eccentricity level for one revolution of the disk 12. Referring to FIGS. 1A and 1B, note that at points c and d, the track 100 and the rotational path 102 are aligned for zero eccentricity. At points a and b, the track 100 and the rotational path 102 are most misaligned for a maximum level of eccentricity in opposite directions. Thus, the plot of eccentricity for one revolution of the disk 12 is a sine wave as illustrated in FIG. 1B.
FIG. 1C illustrates tracks and sectors of the disk 12. Tx indicates a track number, and Sy indicates a sector number for each sector. Thus, the disk 12 of FIG. 1C has three tracks with eight sectors in each track for clarity and simplicity of illustration and description. However, a typical disk has more numerous tracks and sectors.
Referring to FIG. 1D, each sector 104 of the disk 12 includes respective servo information 106 and respective data 108. Servo information 106 indicates the identification and location of the sector 104.
Because of the eccentricity of the disk 12 with respect to the spindle motor, the head for reading the servo information is not aligned to the tracks of the disk 12. Such misalignment results in servo time jitter in the servo signal detected by the head from the track of the disc 12. Such servo time jitter refers to change in timing of the servo signal detected by the head from the track of the disc 12. Referring to FIGS. 1A and 1B, the servo time jitter is worst for sectors located near points a and b, and the servo time jitter is least for sectors located near points c and d.
Such servo time jitter needs to be compensated in order to accurately extract the servo information from the servo signal. In the prior art, servo time jitter values are measured and stored in a table for each sector during manufacture of the HDD. During operation of the HDD, each servo time jitter value is read from the table, and the start time of a servo gate pulse corresponding to each sector is adjusted according to the servo time jitter value.
However, the eccentricity (and thus the servo time jitter value) would change from unexpected sources such as an external impact on the HDD. The servo time jitter values stored in the table would not account for such changed eccentricity. In addition, time is needed to measure the servo time jitter value during manufacture of the HDD, and memory capacity is needed for storing the table of the servo time jitter values.